1. Field of the Invention
The present invention relates to a positive-electrode material for nonaqueous-electrolyte secondary batteries, a manufacturing method therefor, and a nonaqueous-electrolyte secondary battery using said positive-electrode material.
2. Description of the Related Art
In recent years, with the spread of portable electronic equipment, such as cell phones and notebook-sized personal computers, development of a small and lightweight nonaqueous-electrolyte secondary battery having a high energy density has been strongly desired. Also, development of a high-output secondary battery as a battery for electric vehicles, such as hybrid vehicles, has been strongly desired.
As a secondary battery satisfying such demands, there can be mentioned a lithium-ion secondary battery.
This lithium-ion secondary battery uses a material capable of desorption and insertion of lithium as an active material for a negative electrode and a positive electrode.
At present, research and development of such lithium-ion secondary battery are being actively conducted, and particularly, since a 4V class high voltage can be achieved by a lithium-ion secondary battery using lithium metal composite oxide having a layered or spinel structure as a positive-electrode material, the commercialization thereof as a battery having a high energy density is progressing.
Examples of active materials which have been proposed until now include lithium-cobalt composite oxide (LiCoO2), which is relatively easily synthesized; lithium-nickel composite oxide (LiNiO2), in which nickel, more inexpensive than cobalt, is contained; lithium-nickel-cobalt-manganese composite oxide (LiNi1/3Co1/3Mn1/3O2); and lithium-manganese composite oxide (LiMn2O4), in which manganese is contained.
Among these, lithium-nickel composite oxide and lithium-nickel-cobalt-manganese composite oxide have been highlighted as a material demonstrating excellent battery characteristics, such as having excellent cycle characteristics and providing high output with low resistance, and furthermore, in recent years, lower resistance, which is necessary for achieving higher output, has been regarded as important.
As a method for improving such battery characteristics, addition of different elements is applied, and particularly, it is supposed that a high-valent transition metal, such as W, Mo, Nb, Ta, or Re, is useful.
For example, Japanese Patent Application Laid-Open No. 2009-289726 proposes a lithium transition metal compound powder for use as a positive-electrode material for lithium secondary batteries, the lithium transition metal compound powder containing 0.1 to 5 mol % of at least one element selected from Mo, W, Nb, Ta, and Re, with respect to a total molar amount of Mn, Ni, and Co, and describes that an atomic ratio of a total of Mo, W, Nb, Ta, and Re to a total of metallic elements other than Li and Mo, W, Nb, Ta, Re, on the surface of primary particles, is preferably five or more times than the atomic ratio of the whole of the primary particles.
According to this proposal, there can be provided low cost, high safety and high load characteristics, as well as improvement in powder handling, in the lithium transition metal compound powder for use as a positive-electrode material for lithium secondary batteries.
The above-mentioned lithium transition metal compound powder is obtained in such a manner that a raw material is pulverized in a liquid medium, and then a slurry in which the raw material is uniformly dispersed is sprayed and dried, and baked. Therefore, there is a problem that a part of the different elements, such as Mo, W, Nb, Ta, and Re, substitutes for Ni which has been arranged in layers, whereby battery characteristics, such as battery capacity and cycle characteristics, decrease.
Japanese Patent Application Laid-Open No. 2005-251716 proposes a positive-electrode active material for nonaqueous-electrolyte secondary batteries, the positive-electrode active material comprising at least lithium transition metal composite oxide having a layered structure, wherein the lithium transition metal composite oxide exists in a form of particles comprising either or both of primary particles and secondary particles composed of aggregation of the primary particles, and has, at least on the surface of said particles, a compound comprising at least one selected from the group consisting of molybdenum, vanadium, tungsten, boron, and fluorine, and Patent Literature 2 describes that the positive-electrode active material for nonaqueous-electrolyte secondary batteries achieves excellent battery characteristics even under severer environment conditions for use.
Japanese Patent Application Laid-Open No. 2005-251716 particularly discloses that, since the positive-electrode active material has, on the surface of the particles, a compound comprising at least one selected from the group consisting of molybdenum, vanadium, tungsten, boron, and fluorine, electrical conductivity is improved, whereby the initial characteristics are improved without loss of improvement in thermal stability, load characteristics, and output characteristics.
However, Japanese Patent Application Laid-Open No. 2005-251716 discloses that the effect of the selected addition element exists in improvement in initial characteristics, that is, initial discharge capacity and initial efficiency, and the effect on output characteristics is dependent on the aspect ratio of primary particles.
According to the disclosed manufacturing method, there is a problem that, the addition element is mixed and baked with a hydroxide which has been heat-treated simultaneously with a lithium compound, and therefore a part of the addition element substitutes for nickel which has been arranged in layers, whereby battery characteristics decrease.
Furthermore, there has been proposed an improvement achieved by forming a compound layer made of lithium and a different element on the surfaces of positive-electrode active material particles. For example, Japanese Patent Application Laid-Open No. 2002-75367 proposes a positive-electrode active material for lithium secondary batteries, the positive-electrode active material having a surface layer containing Li and at least one selected from the group consisting of Mo and W on the surface of lithium composite oxide capable of inclusion and desorption of Li ions.
This proposal aims at providing a positive-electrode active material having good thermal stability without greatly degrading high initial discharge capacity in order to achieve a lithium-ion secondary battery having higher capacity and higher energy density and having a large size, but Japanese Patent Application Laid-Open No. 2002-75367 does not mention any improvement in output characteristics.
On the other hand, there has been proposed an attempt to blend a material having an improving effect on battery characteristics into component materials of a battery without improvement of an active material itself.
For example, Japanese Patent Application Laid-Open No. 2008-285388 proposes a lithium-ion conductivity improving material comprising a metal oxide carrier and a lithium ion conductive group or a lithium ion conductive metal oxide, the lithium ion conductive group and the lithium ion conductive metal oxide each being supported on the metal oxide carrier and having higher acidity than the metal oxide carrier has.
According to this proposal, the use of the conductivity improving material allows lithium ion conductivity to be effectively improved, thereby reducing lithium ion resistance of a battery.
However, as Japanese Patent Application Laid-Open No. 2008-285388 describes that, by addition of the lithium-ion conductivity improving material to any one of a positive-electrode layer, a negative electrode layer and a separator, lithium ion conductivity is improved, this proposal aims at improvement in conductivity of lithium ions between battery component materials, and the proposal does not aim at reduction in resistance in charge and discharge of a positive-electrode active material itself. Furthermore, it cannot be said that the improving effect on lithium ion resistance disclosed in Examples is satisfactory.
In view of such problems, the present invention aims to provide a positive-electrode material mixture for nonaqueous-electrolyte secondary batteries, the positive-electrode material mixture achieving high output as well as high capacity when used for a positive-electrode.